Bad Circuit Design 1 - Dueling Current Sources
This topic is discussed from the bottom of page 867 to page 871; however, it is such a common problem
that additional discussion is provided here.
Putting two current sources in series, as seen below, clearly isn’t possible in a real circuit.
The transistor(s) used to implement one of the current sources will triode to ensure a consistent
current flowing from power to ground.
In the transistor implementation of this circuit, below, the PMOS will triode so that the current
flowing in the circuit goes to 10 uA (and thus Vx will move towards VDD). It’s important to note
that you can’t make the current flowing in the PMOS precisely equal the current flowing in the NMOS
without one of the devices operating in triode. Yes, once in a while it will work but it’s not manufacturable.
Don’t let SPICE trick you into thinking that by tweaking the length and the width of the devices you
can make the currents equal (yikes! ;-) and keep both devices in saturation (always). In real circuits the
performance will vary with process shifts, temperature, and changes in the power supply voltage (VDD).
One more comment before moving on, you may see this topology on the output of an op-amp, see
Fig. 24.44 for example. If we connect both inputs of that (Fig. 24.44) op-amp to VDD/2 the output
of the op-amp will move towards either VDD or ground because of the issues we just mentioned.
By adding negative feedback around the op-amp, output back to the - input, the “effective” bias voltages
on the gates of MOP and MON are adjusted so that the currents in MON and MOP are equal and the
output voltage is set…but only by adding the feedback.
Next consider the diff-amp with current source loads seen below. It doesn’t matter what voltages
we apply to the gates of M1 and M2 it’s impossible, in a real circuit, for the 20 uA currents to sum to
precisely 40 uA. Yes, once in a great while, at a specific temperature or process run we may end up
with a
circuit that doesn’t have transistors operating in the triode region but in any
case this is bad design!
Feedback around the circuit back to Vin will do nothing to help and your circuit will not work!
Before giving some transistor-level examples of this kind of bad design we should point out that if we
employ a circuit to measure Vout and adjust either the 40 uA or 20 uA current sources we can get this
design to work (the currents on the top to sum to the current on the bottom). For fully-differential designs
this additional circuit is called a common-mode feedback circuit.
Examples of this problem are seen below. Besides op-amps you might see this issue in a delay element
found in a delay line or a voltage-controlled oscillator (see Fig. 19.57 but remove the two gate-drain
connected NMOS in the delay element…baddddd!)
It’s easy to show other examples, e.g., connect the gates of M5/M6 to Vbias4 instead of to the
drains of the PMOS/NMOS on the left side of the folded cascade stack in Fig. 22.5.
To avoid bad design (dueling current sources) use a current mirror load (M3/M4 in Fig. 22.6) or a
common-mode feedback circuit (discussed from the bottom of page 867 to page 871).
Finally, in Fig. 21.21 for example, this problem (one current source dueling with another) is avoided by
adding a “Big” resistor in between the gate and drain of M1. This resistor makes M1 appear, for DC, as
a gate drain connected device (remember that for nano-CMOS there is a DC gate current so the resistor
can’t be too large) so M1 can sink the current sourced by M2. The “Big” capacitor blocks the DC gate
voltage of M1 from the AC input. Notice how we are not using voltage sources for biasing. This (using
voltage sources for biasing) is discussed in bad design 2!